It would depend on the nature of what evidence is sought, not because of any law, but because of the nature of the evidence. For example, if he's subpoenaing his cellphone provider to provide records of a call via his cellphone, than I do not see the point as cellphones store incoming and outgoing calls as well as the duration of the call. No phone company testimony is required because they don't have the evidence, the defense does.
If he's trying to establish he was outside of an area at the time of the murder, it could be important to know which cell tower was pinged during the call as they can give a rough location. (If you're within the same service area of the cell during the time of the murder, it might not work, as establishing a location via cell is not precisely accurate, though it can be narrowed, but it could also be spoofed.). In this case, the phone company would be compelled to testify because it's their equipment that is handling the evidence and they would need to explain it to the court.
If it's his landline and he had placed a service call to that provider, the phone company would record who drove out to the house, what time he started and ended his service and if the defendant was the one who let him in or noticed him in some way that could prove alibi. Again, these are the company's records, not the defense, so they would need to testify to the records and their storage and nature and explain it to the court.
Finally, if the witness is used by the prosecution, the defense has the right to examine all evidence against him, so he would be allowed to talk to the phone company about everything and establish what the witness will say and how to best defend against that testimony.
As a general rule, witness testimony is considered evidence and is generally divided into expert and fact witnesses. The latter is limited only to testifying to what they observed. Expert witnesses can give opinion based on their area of expertise. An expert in automotive maitenance and general knowledge witness cannot testify to say... internet protocol... but they can testify to why a set of tire marks at the scene of the crime could not possibly be made by the Metallic Mint Green Buick Skylark the defendants were seen driving away in and later arrested in... (See "My Cousin Vinny" because it is awesome).